PET Imaging, or positron emitter tomography, starts with the administration (e.g., ingestion or inhalation) of a radiopharmaceutical into a patient, and, in time, the physical and bio-molecular properties of the agent concentrates at specific locations in the human body. The actual spatial distribution, the intensity of the point or region of accumulation, and the kinetics of the process from administration to capture to eventually elimination are all elements that may have a clinical significance. During this whole process, the positron emitter attached to the pharmaceutical agent will emit positrons according to the physical properties of the isotope (i.e., half-life, branching ratio, etc. . . . ). Each emitted positron will eventually interact with an electron of the object to get annihilated and produce two gamma rays at 511 keV at substantially 180 degree apart.
By detecting these two gamma rays, and drawing a line between their locations or line-of-response, the likely location of the original disintegration can be retrieved. While this process only identifies a line of possible interaction, by accumulating a large number of these lines, and through a tomographic reconstruction process, the original distribution can be estimated. In addition to the location of the two scintillation events, if accurate timing (i.e., few hundred picoseconds) is available, time-of-flight calculation can add more information on the likely position of the event along the line.
Limitations in the timing resolution of the scanner will determine the accuracy of the positioning along this line. Further, limitations in the determination of the location of the original scintillation events will determine the ultimate spatial resolution of the scanner. Also, the specific characteristics of the isotope (i.e., energy of the positron) will also contribute, via positron range and co-linearity of the two gamma rays, to the determination of the spatial resolution of the specific agent.
The above process needs to be repeated for a large number of events. While every case needs to be analyzed to determine how many counts (paired events) are required to support the imaging tasks, current practice dictates that a typical 100 cm long, FDG (fluoro-deoxyglucose) study should accumulate a few 100 millions counts. The time it takes to accumulate this number of counts is determined by the injected dose and the sensitivity and counting capacity of the scanner.
The PET scanner is typically substantially cylindrical to be able to capture as much as possible of the radiation which should be, by definition, isotropic. Since the opposite detection of two gamma rays is necessary to create an event, the sensitivity is approximately the square of the solid angle created by the detector arrangement. For example, the use of a partial ring and rotation to capture the missing angles is possible, but has severe consequences regarding the overall sensitivity. From the cylindrical geometry, in which all gamma rays included in a plane would have a chance to interact with the detector, an increase of the axial dimension would have a very beneficial effect on the sensitivity or ability to capture the radiation, leading to the ultimate design of a sphere, in which all gamma rays would have the opportunity to be detected. However, the spherical design is not feasible due to the large size and high costs required for creating a spherical PET scanner suitable for application on humans. Therefore, the modern PET scanner includes a cylindrical geometry with the axial extent as a variable.
Once the overall geometry of the scanner is determined, the next challenge is to dispose as much scintillating material as possible in the gamma ray path to stop and convert as many gamma rays as possible into light. Two directions of optimization are considered in this process. First, the “in-plane” sensitivity necessitates that as much crystal as possible (i.e., crystal thickness) be disposed within a PET detector. Second, for a given crystal thickness, the axial length of the detector-cylinder defines the overall system sensitivity, which is approximately proportional to the square of the axial length (the solid angle subtended by a point in the middle of a cylinder). Additionally, practical cost considerations are unavoidably part of the optimization process.
While it is generally desirable to obtain as large of an axial length as possible for sensitivity, clinical needs may impose an additional set of constraints. For example, some clinical tests may require a PET scanner to cover an entire organ such as the lung or multiple organs such as the heart and carotids. Therefore, the goal of designing a PET scanner is to optimize the cost, sensitivity, and axial length of the PET scanner.